There is known a method of producing spherical silica particles of high mono-dispersion by subjecting, for example, an alkoxide of silicon or the like, to hydrolysis and polycondensation. A method of producing metal oxide particles by subjecting an organic metal compound (e.g. metal alkoxide) as a raw material to polycondensation in a water-containing solvent, is generally called sol-gel processing. By using, as raw materials, an alkoxide of silicon and an alkoxide of a metal other than silica, silica-based composite oxide particles can be produced by the sol-gel processing. As the silica-based composite oxide particles, there are mentioned, for example, particles of silica-titania, silica-alumina, silica-zirconia, etc.
The silica-based composite oxide particles produced by the sol-gel processing are given characteristic properties depending upon the kind of the metal oxide contained in the particles together with silica. These characteristics are unobtainable with particles produced with silica alone.
The particles having such characteristics are in use in various applications. For example, by changing the mixing ratio of silica and a metal oxide other than silica, there can be obtained particles of desired refractive index while the optical transparency of the particles is maintained. By applying this technique, a transparent composite resin or a dental composite resin are obtained.
These composite resins, when using particles having a refractive index identical with those of the resins as filler, can show improved properties in mechanical strength, low thermal expansion, etc. while maintaining the visual transparency.
It is advantageous that the filler compounded in dental composite resin has radiopacity in addition to the above-mentioned transparency, from the standpoint of easy diagnosis of prognosis. Hence, as the filler compounded in dental composite resin, there are ordinarily used silica-based composite oxide particles which are a composite of silica and other metal oxide. As the other metal oxide in the silica-based composite oxide particles, there are preferred titania, zirconia, barium oxide, etc. all having Radiopacity. By controlling the compounding ratio of such a metal oxide to make the refractive index of filler identical with that of resin, there can be obtained a dental filler having both transparency and Radiopacity.
Meanwhile, in producing silica-based composite oxide particles by the above-mentioned sol-gel processing, there has ordinarily been a problem that silica-based composite oxide particles formed tend to coagulate. Further, fresh silica-based composite oxide particles are incessantly formed with the passage of time and this causes a problem that the diameters of the particles formed become non-uniform. These phenomena are striking as the content of the metal oxide other than silica in composite oxide particles is higher. This makes it difficult to obtain silica-based composite oxide particles of high mono-dispersion and uniform particle diameter.
For producing silica-based composite oxide particles of high mono-dispersion, there was proposed a method of using water-containing acetonitrile in the reaction mixture of sol-gel processing (reference is made to Patent Literature 1). In the method, the concentration of acetonitrile in the reaction mixture is at least 20% by mass. The method is very effective for solving the above problems and, in fact, can efficiently produce silica-based composite oxide particles of high mono-dispersion.